Contact lenses having one or more colorants dispersed in the lens or printed on the lens are in high demand. These colored contact lenses may enhance the natural beauty of the eye, or provide unique patterns on the iris of the wearer, or provide non-cosmetic patterns or marks, such as rotation marks, inversion marks, product/brand codes, lot numbers, “DEMO” lenses, and the like, which are of benefits to wearers, eye-care practitioners and manufacturers.
A known method of printing inks onto contact lens involves ink transfer printing. According to this method the colored film to be transferred to the lens is first deposited on a surface of the mold wherein the lens is subsequently formed. The pattern of the liquid colored film can be provided by etching or lasering a metal or ceramic plate. The etched pattern is then filled with an appropriate ink, leveled out with a suitable doctoring device, and a soft silicone rubber transfer stamp or pad impressed thereupon to transfer the pattern on the pad's surface. The pad containing the image is then brought to the lens forming mold and depressed in the correct place to transfer the pattern thus creating the colored film. The colored film may then be subjected to a partial or full cure. Thereafter, lens forming material is added to the mold and actinically or thermally cured to form the contact lens whereby the colored film detaches from the molding surface and becomes integral with the body of the contact lens.
Several organic solvent based inks have been used for ink transfer printing techniques of color images on a contact lens. In fact, due to the increased demand of colored contact lenses which requires mass production of the same, it is desirable to use an ink which is soluble in water. The use of a water based ink allows simplifying the automated production of colored contact lenses, resulting in more environmentally friendly and inexpensive products.
When employing a water based ink for ink transfer printing of color images on a contact lens the degree of ink transfer as well as smearing and delamination of the same is found to depend on several factors. Among them, the time required to transfer the ink from the cliché to the mold; the contact lens material, and the ink formulation.
In particular, it has been observed that the longer time a water based ink is kept hanging on a pad after picking up the ink from the cliché and before depositing it on the mold results in a degraded image quality. This has been attributed to the drying of the ink on the pad. Even for very short hanging time (1 to 5 seconds) the ink does begin to dry and eventually a long enough delay causes the ink to dry completely on the pad and eliminate the ink transfer or severely degrades the image when it is transferred to the mold.
In addition when the contact lens forming material is a water based monomer the mechanical strength of the lenses is surprisingly found to be strictly correlated with the water content in the ink and in the air.
Therefore there is a need to develop a water based ink transfer printing method for making colored contact lenses with improved image quality without compromising the lenses quality.